Iron and silver are present, sometimes in large quantities, in numerous effluents and, in particular, in photographic effluents.
The elimination of heavy metals in effluents before their disposal to the sewage plant is becoming a necessity in order to satisfy the standards for waste laid down for better protection of the environment. Silver is one of the metals envisaged by these standards. Various techniques have been described for the elimination of silver. Precipitation in the form of hydroxide or sulphur, electrolysis, calcination and flocculation remain the most common techniques. Flocculation uses thickeners such as silicates, and/or flocculents.
For the elimination of metals, the use of ion exchange resins or zeolites has also been described, but this technique appears to be applicable only to effluents which contain only small quantities of silver. Furthermore, saturation of the resin or zeolite occurs rapidly. Finally, zeolite can, apart from silver, adsorb organic substances which contribute to a reduction in its efficacy. Treatment by zeolite is described by J E Garcia Hermandez et al in "Environ. Pollut.", 76 (3), 219-223 (1992) and treatment by resins is described by M Diaz et al in "J. Metall" 39 (7) 42-44 (1987).
An aluminium and magnesium hydroxycarbonate designated by the name of hydrotalcite, existing in the natural state, but also prepared by synthesis, has been used as an adsorbent in order to extract metallic complexes from aqueous solutions in which they are found.
U.S. Pat. No. 4,752,397 describes a method for purifying an aqueous effluent by passing it through a calcinated hydrotalcite. The substances (metallic complexes) adsorbed on the hydrotalcite are then desorbed.
U.S. Pat. No. 4,867,882 describes a method for eliminating from an aqueous effluent anionic complexes, for example a complex of silver dithiosulphate or copper-EDTA, by passing this effluent over a hydrotalcite.
Conventionally, hydrotalcite is used in the form of a fine powder, the elimination of which by decanting and filtration is difficult. Hydrotalcites are mixed hydroxycarbonates, notably of magnesium and aluminium. They are usually prepared by co-precipitating magnesium and aluminium hydroxides at a pH around 10 in order to obtain a powder hydrotalcite. It is also possible to calcinate the hydrotalcite powder. This powder is an efficacious adsorbent for ions and complexes of heavy metals.